Forensic document experts may be asked to examine questioned items written with spray paint on vertical surfaces. In such cases, they may not dispose of reference material written under similar conditions. A critical question follows: is conventional reference material (e.g. executed with a ballpoint pen on a paper placed horizontally) suitable for comparison? In order to investigate the influence of this particular writing condition, samples written with spray paint on vertical surfaces were compared to samples written on paper by a same writer. This comparison was carried out on the writings of 27 volunteers. The results have shown that the amount of similarities between both writing conditions was strongly dependent on the writer. Overall, while some similarities in style (script, cursive), shape and construction of letters were observed, several discordances in connections, slant, spacing, size and proportions were highlighted between both writing conditions. Therefore, the findings of this study stress out the need for reference material produced under similar conditions when examining questioned spray paint writings on vertical surfaces.